The structural changes of the title compound during heating and under conditions of catalytic conversion of isobutyric acid to methacrylic acid were followed in situ by powder X-ray diffraction under continuous control of its activity. The results were verified by a postmortem phase analysis of practical supported catalyst samples used in kinetic reactors. The activity of the catalyst is correlated with its dehydrated form, A new cubic phase of a water-free vanadyl salt of the heteropolyacid (HPA) was found to be connected to a maximum conversion, This phase is isostructural to the unsubstituted anhydrous alkali-3-HPA salts and is metastable at ambient conditions with respect to rehydration. The catalyst material as a whole is metastable at any temperature above the onset of conversion with respect to a partially reversible decomposition into MoO3 and amorphous other components, Restructuring into crystalline forms of HPA is possible from the deactivated material upon dissolution and recrystallization at 323 K. In situ UV-VIS data and X-ray diffraction show the complete self-reorganization of the MoO3 phase and the amorphous V and P compounds into new Keggin anions indicating the possible living nature of the catalyst under reaction conditions which enable extended lifetimes beyond the stability limits found in the present in sih X-ray diffraction experiments.